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Telegram/lib_rpl/rpl/distinct_until_changed.h

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+// This file is part of Desktop App Toolkit,
+// a set of libraries for developing nice desktop applications.
+//
+// For license and copyright information please follow this link:
+// https://github.com/desktop-app/legal/blob/master/LEGAL
+//
+#pragma once
+
+#include <rpl/producer.h>
+#include "base/optional.h"
+
+namespace rpl {
+namespace details {
+
+class distinct_until_changed_helper {
+public:
+	template <typename Value, typename Error, typename Generator>
+	auto operator()(
+			producer<Value, Error, Generator> &&initial) const {
+		return make_producer<Value, Error>([
+			initial = std::move(initial)
+		](const auto &consumer) mutable {
+			auto previous = consumer.template make_state<
+				std::optional<Value>
+			>();
+			return std::move(initial).start(
+				[consumer, previous](auto &&value) {
+					if (!(*previous) || (**previous) != value) {
+						*previous = value;
+						consumer.put_next_forward(std::forward<decltype(value)>(value));
+					}
+				}, [consumer](auto &&error) {
+					consumer.put_error_forward(std::forward<decltype(error)>(error));
+				}, [consumer] {
+					consumer.put_done();
+				});
+		});
+	}
+
+};
+
+} // namespace details
+
+inline auto distinct_until_changed()
+-> details::distinct_until_changed_helper {
+	return details::distinct_until_changed_helper();
+}
+
+} // namespace rpl